A long-standing problem in the treatment of glioblastoma (GBM), the most common and deadly primary brain cancer, is delivery of therapeutics to brain-invading tumor cells outside of the area that is safe for surgical removal. Recent evidence indicates that reactive macrophages, microglia, and other immune cells infiltrate brain-invaded GBM regions and frequently become tumor-supporting cells. Establishing strategies for effective therapeutic delivery to the tumor and tumor-supporting cells, which contribute to this residual, invasive tumor microenvironment (TME) is an important unmet clinical need. To address this need, we have developed biodegradable nanoparticles (NPs) with specialized surface coatings that diffuse rapidly and target remote cells within the brain. We call these decreased adhesivity receptor-targeted nano-formulations, ?DARTs?. DARTs can serve as advanced brain delivery tools to improve therapeutic efficacy and decrease off-target toxicities ? both critical hurdles for safe, effective treatments in the brain. A promising cell portal for targeted GBM therapeutics is the TNF receptor superfamily member, fibroblast growth factor inducible-14 (Fn14). Fn14 is minimally expressed in the healthy brain, moderately expressed in the GBM core and most importantly, highly expressed in the brain-invading GBM cells. More recently, we have discovered high levels of Fn14 on TAMs with tumor- supporting (M2-like) features, and elevated Fn14 expression leads to aggressive GBMs with shorter host survival. These new findings coupled with the promise of DARTs, motivate the studies in this proposal. Our central hypothesis is that Fn14 plays specific tumor-supporting roles in the invasive GBM microenvironment and Fn14 DARTs will selectively target, traffic within, and deliver drugs to Fn14-positive (Fn14+) tumor cells and TAMs. In Aim 1, we will investigate DART trafficking and cellular dynamics in Fn14+ and Fn14- TME cells to better understand the mechanisms governing NP localization within these cells and distribution in specific cell populations. This information will help us optimize the DART formulations to improve selectivity, cellular retention, and minimize off target toxicities. In Aim 2, we will explore the potential impact of Fn14 DARTs on the TME and Fn14 related therapeutic opportunities through investigations of cellular activation phenotypes and functional variations present in Fn14+/+ or -/- tumor-host pairings. In Aim 3, we will couple our ongoing efforts with work from this project to evaluate the efficacy of therapeutic delivery to TME cells via CED of Fn14 DARTs. This project will develop a new anti-GBM therapeutic strategy designed to address the invasive GBM microenvironment, and will help refine the approach for future canine and human clinical trials.